The present invention relates to image coding apparatuses and image coding methods, image decoding apparatuses and image decoding methods, and data recording media and, more particularly, to a coding process and a decoding process for performing recording or transmission of image signals with less bit number without degrading the image quality, and to a recording medium containing a program for realizing the coding process or the decoding process.
Conventional image coding processes are broadly divided into two of coding processes performed in block units, represented by MPEG2-based coding methods, and coding processes performed in pixel units, such as differential pulse code modulation (DPCM: Differential Pulse Code Modulation).
The coding process in block units is a method in which a single image display region is divided into plural blocks and a coding process for an image signal that is input (hereinafter referred to as an input image signal) is performed block by block. In this case, the single image display region corresponds to a single display screen in the MPEG2-based coding process, and in an MPEG4-based coding process, it corresponds to a display region having a shape and a size corresponding to each object on a single display screen. Further, each of the above-mentioned blocks is a display region composed of a prescribed number of pixels within the single image display region, and a rectangle shape which is easily processed is used as the shape of the block in many cases.
As described above, in the coding method in which a coding process for an input image signal is performed block by block, the coding process for the input image signal corresponding to a single image display region is completed in each block. Therefore, there is the advantage that, even though a transmission error occurs when a coded image signal obtained by performing the coding process for the input image signal is transmitted, influence of the error can be converged in each block.
However, the block-by-block coding method has the following drawbacks.
Since the coding process for the input image signal is completed in each block in the block-by-block coding method, it is difficult to use an inter-block pixel correlation, that is, a correlation of pixel values which are present in different blocks, in the coding process.
Further, in a predictive coding method for an image signal, a pixel value of a coding target pixel being a target of coding (a coding target pixel value) is predicted with reference to pixel values of plural coded pixels which have previously been coded (coded pixel values), and the coding target pixel value is adaptively coded using the predicted pixel value. In this predictive coding method, however, when the coding process is performed block by block, the coded pixel values to be referred to when coding the coding target pixel value are limited to pixels within each block, so that the reference coded pixel values are small in number. Therefore, the accuracy of the predicted value of the coding target pixel is reduced, and the coding efficiency is not increased very much.
On the other hand, the coding method in pixel units is a method in which an input image signal is coded pixel by pixel, and in this coding method, it is possible to change the coding process for the input image signal pixel by pixel. Therefore, when this coding method includes a universal coding process, such as adaptive arithmetic coding in which code words are automatically updated pixel by pixel adaptively to the characteristics of the input image signal, an image signal having any characteristic can be coded with a significantly high coding efficiency.
However, since, on the decoding side, a coded image signal obtained by the pixel-by-pixel coding method including the universal coding process is subjected to a decoding process in which code words are updated in the same manner as on the coding side, when a transmission error occurs when the coded image signal is transmitted, the state in which the decoding process for the coded image signal cannot be carried out accurately because of the influence of the transmission error on the decoding side, continues long.
By the way, the block-by-block coding method and the pixel-by-pixel coding method can be combined, and in a coding method in which these coding methods are combined (hereinafter referring to this coding method as a combination coding method for explanation), code words can be adaptively changed for each pixel, and the influence of transmission error can be converged in each block, whereby a coding process with a high coding efficiency, such as adaptive arithmetic coding, can be performed while suppressing the influence of transmission error.
A description is now given of this combination coding method.
FIG. 13(a) shows the state in which a single frame screen is divided into a plurality of rectangle blocks, and FIG. 13(b) shows arrangement of pixels in blocks, especially in a coding target block being a target of coding and blocks in the vicinity of the coding target block. Needless to say, these pixels are arranged in matrix along horizontal scanning lines in the single frame screen.
In the figures, FG denotes a screen corresponding to a single frame, B1 denotes a coded block in which a coding process for an image signal has already been performed, Bx denotes a coding target block being a target of coding, and B0 denotes an uncoded block in which a coding process for an image signal has not been performed. When no distinction is made between blocks, blocks are denoted by B. BLu, BLs, BLh, and BLm denote upper, lower, left, and right boundaries of the coding target block in the single frame screen, respectively. A solid line circle shows a coded pixel whose pixel value has already been coded, and a dotted line circle shows an uncoded pixel whose pixel value has not been coded yet. Each block B is an image display region comprising 4xc3x974 pixels, in the single frame screen FG.
FIG. 14 shows positional relationships between a coding target pixel Px to be coded and peripheral pixels P0xcx9cP9 surrounding the coding target pixel, and the pixel values of these peripheral pixels P0xcx9cP9 are referred to when the pixel value of the coding target pixel Px is predicted, so that these pixels are called reference pixels hereinafter.
The reference pixels P8 and P9 are pixels which are positioned in the same horizontal scanning line as the coding target pixel Px, and the reference pixels P9 and P8 are positioned one pixel and two pixels before the coding target pixel Px, respectively. The positions of the reference pixels P5 and P1 in the horizontal direction on the single frame screen FG correspond to the position of the coding target pixel Px, and the reference pixels P5 and P1 are positioned in a horizontal scanning line by one pixel and two pixels upper than the coding target pixel Px, respectively. Further, the reference pixels P3, P4, P6, and P7 are pixels which are positioned in the same horizontal scanning line as the reference pixel P5, the reference pixels P4 and P3 are positioned one pixel and two pixels before the coding target pixel Px, respectively, and the reference pixels P6 and P7 are positioned one pixel and two pixels after the coding target pixel Px, respectively. Further, the reference pixels P0 and P2 are pixels which are positioned in the same horizontal scanning line as the reference pixel P1, the reference pixel P0 is positioned one pixel before the reference pixel P1, and the reference pixel P2 is positioned one pixel after the reference pixel P1.
In the combination coding method, initially, an image signal corresponding to the single frame screen FG is divided correspondingly to plural blocks B constituting the single frame screen as shown in FIG. 13(a) and FIG. 13(b), and a coding process for the divided image signal is performed block by block.
This block-by-block coding process is completed by performing a horizontal process in which pixel values of pixels are successively coded from the left to the right along each horizontal pixel line in a block B, for all of the horizontal pixel lines in the block from the uppermost line to the lower most line.
In this coding process, as shown in FIG. 14, the pixel value of the coding target pixel Px is adaptively predicted from the pixel values of the reference pixels P0xcx9cP9 positioned in the vicinity of the coding target pixel, and code words used for the coding process of the coding target pixel are adaptively selected according to a predicted value obtained by the prediction.
Therefore, in the combination coding method, the influence of transmission error on the decoding side can be converged in each block, and the coding efficiency can be improved as compared with the simple block-by-block coding process.
FIG. 16(a), FIG. 16(b), and FIG. 17 are diagrams for explaining a combination decoding method corresponding to the above-mentioned combination coding method, and in the figures, Bxe2x80x2 denotes each block in a single frame, Bxxe2x80x2 denotes a decoding target block, B1xe2x80x2 denotes an already decoded block, B0xe2x80x2 denotes an undecoded block, BLuxe2x80x2, BLsxe2x80x2, BLhxe2x80x2, and Blmxe2x80x2 denote block boundaries of the decoding target block Bxxe2x80x2 at the upper, lower, left, and right sides, respectively, and P0xe2x80x2xcx9cP9xe2x80x2 denote reference pixels corresponding to a decoding target pixel Pxxe2x80x2. In this case, the arrangement of the reference pixels P0xe2x80x2xcx9cP9xe2x80x2 with respect to the decoding target pixel Pxxe2x80x2 is identical to that described for the coding process shown in FIG. 13(a), FIG. 13(b), and FIG. 14.
In the combination decoding method, initially, an image signal corresponding to a single frame screen FGxe2x80x2 is divided correspondingly to plural blocks Bxe2x80x2 constituting the single frame screen as shown in FIG. 16(a) and FIG. 16(b), and a decoding process for the divided image signal is performed block by block.
This block-by-block decoding process is completed by performing a horizontal process in which pixel values of pixels are successively decoded from the left to the right along each horizontal pixel line in a block Bxe2x80x2, for all of the horizontal pixel lines in the block from the uppermost line to the lower most line.
In this decoding process, as shown in FIG. 17, the pixel value of the decoding target pixel Pxxe2x80x2 is adaptively predicted from the pixel values of the reference pixels P0xe2x80x2xcx9cP9xe2x80x2 positioned in the vicinity of the decoding target pixel, and code words used for the decoding process of the decoding target pixel Pxxe2x80x2 are adaptively selected according to a predicted value obtained by the prediction.
However, the combination coding method in which the block-by-block coding process and the pixel-by-pixel coding process are combined has the following drawbacks.
In this combination coding method, since the coding process proceeds block by block, when the coding target pixel Px abuts the right boundary BLm of the coding target block Bx as shown in FIG. 15, the reference pixels P2, P6, and P7 corresponding to the coding target pixel Px are uncoded pixels.
In this case, when the pixel value of the coding target pixel Px is predicted with reference to the pixel values of the uncoded pixels P2, P6, and P7 and the pixel value of the coding target pixel Px is coded using the predicted value, a coded image signal corresponding to this coding target pixel Px cannot be accurately decoded on the decoding side.
More specifically, in order to accurately decode a coded image signal which is obtained by coding the pixel value of the coding target pixel Px using the predicted value, the predicted value of the decoding target pixel Pxxe2x80x2 used in the decoding process must agree with the predicted value of the coding target pixel Px corresponding to the decoding target pixel Pxxe2x80x2, used in the coding process. In other words, on the coding side, the reference pixel values referred to for generating the predicted value of the coding target pixel Px must completely agree with the reference pixel values referred to for generating the predicted value of the decoding target pixel Pxxe2x80x2 corresponding to the coding target pixel Px.
For this reason, for example, as shown in FIG. 15, when the coding process for the coding target pixel Px is performed, if the predicted value of the coding target pixel Px is generated with reference to the pixel values of the uncoded pixels P2, P6, and P7 among the reference pixels P0xcx9cP9 corresponding to the decoding target pixel Px, as shown in FIG. 18, when the decoding process for the decoding target pixel Pxxe2x80x2 is performed on the decoding side, the predicted value of the decoding target pixel Pxxe2x80x2 is generated with reference to the pixel values of the reference pixels P0xe2x80x2xcx9cP9xe2x80x2 corresponding to the decoding target pixel Pxxe2x80x2, but among the reference pixels P0xe2x80x2xcx9cP9xe2x80x2, pixel values of the undecoded pixels P2xe2x80x2, P6xe2x80x2, and P7xe2x80x2 are not obtained on the decoding side, so that the pixel value of the decoding target pixel Pxxe2x80x2 corresponding to the coding target pixel Px cannot be decoded.
Therefore, in the conventional combination coding method, in order to avoid the problem that the decoding process becomes difficult when uncoded pixels are included in the reference pixels P0xcx9cP9 corresponding to the coding target pixel Px as described above, there is taken a countermeasure in which the predicted value of the coding target pixel Px is generated with the pixel values of the uncoded pixels being regarded as a fixed value which has been previously set (e.g., 0), and the coding process for the coding target pixel Px is performed using this predicted value.
Although the combination coding method with the above-mentioned countermeasure enables the decoding process to be accurately performed for all of the pixels in each block using their predicted values, since the pixel values of the reference pixels being uncoded pixels are uniformly replaced with a fixed value, the correlation of pixel values between the uncoded pixel and the coded pixel is degraded, resulting in the problem that the efficiency in predicting the coding target pixel, i.e., the accuracy of the predicted value of the coding target pixel, is degraded.
The present invention is subjected to solving the above-described problems, and has an object to provide an image processing apparatus and an image processing method, which can combine an adaptive pixel-by-pixel coding process and a block-by-block coding process, without degrading correlation of pixel values between uncoded pixels and coded pixels, with avoiding that decoding of a coded image signal becomes difficult, and to provide a data recording medium in which an image processing program for realizing the image processing method is recorded.
It is another object of the present invention to provide an image processing apparatus and an image processing method, which can perform an accurate decoding process for a coded image signal which has been coded, without degrading an efficiency in predicting coding target pixels, and to provide a data recording medium in which an image processing program for realizing the image processing method is recorded.
An image processing apparatus according to the present invention being an image coding apparatus for successively coding pixel values constituting an image signal on the basis of pixel values of plural peripheral pixels positioned in the vicinity of a coding target pixel, comprises blocking means for blocking an image signal comprising plural pixel values corresponding to a single image display region into blocks each comprising a prescribed number of pixels, and outputting, block by block, the prescribed number of pixel values constituting the image signal in each block; pixel value replacing means for replacing a pixel value of an uncoded pixel among the peripheral pixels with a pseudo pixel value that is obtained from a pixel value of a coded pixel among the peripheral pixels on the basis of a prescribed rule; and coding means for receiving the image signal comprising plural pixel values corresponding to each block, performing, for each block, a coding process in which the respective pixel values are successively coded on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, and outputting an coded image signal.
Since the image coding apparatus thus constructed is provided with the pixel value replacing means for replacing a pixel value of an uncoded pixel among plural peripheral pixels corresponding to a coding target pixel with a pseudo pixel value that is obtained on the basis of a pixel value of a coded pixel among the plural peripheral pixels, when a block-by-block coding process for an image signal corresponding to a single image display region is performed pixel by pixel, with reference to the pixel values of the peripheral pixels in the vicinity of the coding target pixel, even though the coding target pixel abuts on the block boundary and the plural reference peripheral pixels include an uncoded pixel, a pseudo pixel value having an improved correlation with the pixel values of the other peripheral pixels can be referred to as a pixel value of the uncoded pixel. Since this pseudo pixel value is obtained from a pixel value of a coded pixel in the vicinity of the coding target pixel, on the decoder side, with respect to an undecoded pixel to be referred to when a decoding target pixel is decoded, a pseudo pixel value obtained from a pixel value of a decoded pixel can be referred to in place of its pixel value.
Therefore, it is possible to combine an adaptive pixel-by-pixel coding process and a block-by-block coding process without degrading correlation of pixel values between the uncoded pixel and the coding target pixel, with avoiding that decoding of a coded signal becomes difficult on the decoding side.
Thereby, influence of transmission error can be converged block by block, and the coding efficiency can be improved as compared with the simple block-by-block coding process, and further, a decoding process for a coded signal which has been coded without degrading the prediction efficiency of the coding target pixel can be performed accurately.
According to the present invention, in the image coding apparatus, the pixel value replacing means is constructed so as to employ, as a pseudo pixel value of the uncoded pixel, a pixel value of a coded pixel which is positioned at the shortest spatial distance from the uncoded pixel.
Since, in the image coding apparatus thus constructed, the pixel value of the coded pixel which is the nearest to the uncoded pixel is employed as the pseudo pixel value of the uncoded pixel, the correlation between the pseudo pixel value of the uncoded pixel and the pixel values of the peripheral pixels can be improved.
According to the present invention, in the image coding apparatus, the pixel value replacing means is constructed so as to employ, as a pseudo pixel value of the uncoded pixel, a pixel value of a coded pixel which is positioned at the shortest spatial distance from the uncoded pixel and on the same horizontal scanning line as the uncoded pixel.
Since, in the image coding apparatus thus constructed, the pixel value of the coded pixel which is the nearest to the uncoded pixel and on the same horizontal scanning line as the uncoded pixel is employed as the pseudo pixel value of the uncoded pixel, the pseudo pixel value of the uncoded pixel can be obtained by a simple method, such as to hold the pixel value of the cod,ed pixel for a prescribed period of time.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to comprise a prediction value generator for generating a prediction pixel value for the coding target pixel, on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel; and an encoder for coding a difference value between the pixel value of the coding target pixel and the prediction pixel value of the coding target pixel, and outputting the coded difference value, block by block, as a coded image signal.
Since, in the image coding apparatus thus constructed, the difference value between the pixel value of the coding target pixel and the prediction pixel value is coded, the coding efficiency can be increased by reducing the code quantity required for coding the pixel value of the coding target pixel.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to select code words for coding the pixel value of the coding target pixel, on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel.
Since, in the image coding apparatus thus constructed, code words for coding the pixel value of the coding target pixel are selected on the basis of the pixel value of the coded pixel and the pseudo pixel value Of the uncoded pixel, in a coding process for an image signal, a highly efficient coding process can be realized by adaptively changing the code words pixel by pixel.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to select a probability table corresponding to codes for arithmetically coding the pixel value of the coding target pixel, on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, and to perform an arithmetic coding process for the coding target pixel on the basis of the selected probability table.
Since, in the image coding apparatus thus constructed, a probability table corresponding to arithmetic codes for coding the coding target pixel is selected on the basis of the pixel value of the coded pixel and the prediction pixel value of the uncoded pixel, and coding of the pixel value for the coding target pixel is performed, a highly efficient coding process can be performed by adaptively changing the probability table, pixel by pixel, in the arithmetic coding process.
According to the present invention, the image coding apparatus further comprises local decoding means for decoding the pixel value of the coded pixel to generate a local decoded pixel value; wherein the pixel value replacing means is constructed so as to replace the pixel value of the uncoded pixel with a pseudo pixel value that is obtained from a local decoded pixel value corresponding to the coded pixel on the basis of a prescribed rule; and the coding means is constructed so as to perform a non-reversible coding process for the pixel value of the coding target pixel, on the basis of the local decoded pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel.
Since, in the image processing apparatus thus constructed, the non-reversible coding process for the pixel value of the coding target pixel is performed on the basis of the decoded pixel value obtained by decoding the pixel value of the coded pixel, the pixel values of the peripheral pixels to be referred to in the non-reversible coding process for the coding target pixel can be identical to pixel values of peripheral pixels to be referred to in a decoding process for a decoding target pixel, whereby a coded image signal obtained by the non-reversible coding on the basis of the pixel values of the peripheral pixels can be accurately decoded on the decoder side.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to comprise a prediction value generator for generating a prediction pixel value for the coding target pixel, on the basis of the local decoded pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel; and an encoder for coding a difference value between the pixel value of the coding target pixel and the prediction pixel value of the coding target pixel, and outputting the coded difference value, block by block, as a coded image signal.
Since, in the image coding apparatus thus constructed, the prediction pixel value for the coding target pixel is generated from the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, and the difference value between the pixel value of the coding target pixel and its prediction pixel value is coded, the coding efficiency can be improved by reducing the code quantity required for coding the pixel value of the coding target pixel.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to select code words for coding the pixel value of the coding target pixel, on the basis of the local decoded pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel.
Since, in the image coding apparatus thus constructed, code words for coding the pixel value of the coding target pixel are selected on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, in a coding process for an image signal, a highly efficient coding process can be realized by adaptively changing the code words pixel by pixel.
According to the present invention, in the image coding apparatus, the coding means is constructed so as to select a probability table corresponding to codes for arithmetically coding the pixel value of the coding target pixel, on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, and to perform an arithmetic coding process for the coding target pixel on the basis of the selected probability table.
Since, in the image coding apparatus thus constructed, a probability table corresponding to arithmetic codes for coding the coding target pixel is selected on the basis of the pixel value of the coded pixel and the pseudo pixel value of the uncoded pixel, and coding of the pixel value for the coding target pixel is Performed, a highly efficient coding process can be performed by adaptively changing the probability table, pixel by pixel, in he arithmetic coding process.
An image decoding apparatus according to the present invention being an image decoding apparatus for decoding, block by block, a coded image signal which is obtained by performing, for each block comprising a prescribed number of pixels, a process in which pixel values constituting an image signal are successively coded on the basis of pixel values of plural peripheral pixels positioned in the vicinity of a coding target pixel, comprises pixel value replacing means for replacing a pixel value of an undecoded pixel among plural peripheral pixels positioned in the vicinity of a decoding target pixel, with a pseudo pixel value that is obtained from a pixel value of a decoded pixel among the plural peripheral pixels on the basis of a prescribed rule; decoding means for receiving the coded image signal comprising plural pixel values corresponding to each block, performing, block by block, a decoding process in which the respective pixel values are successively decoded on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel, and outputting a decoded image signal corresponding to each block; and inverse blocking means for combining the decoded image signals corresponding to the respective blocks to convert these signals to a decoded image signal having a scanning line structure; wherein the decoded image signal having a scanning line structure is output as a reproduced image signal corresponding to a single image display screen.
Since the image decoding apparatus thus constructed is provided with the pixel value replacing means for replacing a pixel value of an undecoded pixel among plural peripheral pixels positioned in the vicinity of a decoding target pixel, with a pseudo pixel value that is obtained on the basis of a pixel value of a decoded pixel among the plural peripheral pixels, when a block-by-block decoding process for a coded image signal corresponding to a single image display region is performed pixel by pixel, with reference to the pixel values of the peripheral pixels in the vicinity of the decoding target pixel, even though the decoding target pixel abuts on the block boundary and the plural reference peripheral pixels include an undecoded pixel, a pseudo pixel value having an improved correlation with the pixel values of the other peripheral pixels can be referred to as a pixel value of the undecoded pixel.
Therefore, it is possible to realize a decoding method in which an adaptive pixel-by-pixel decoding process and a block-by-block decoding process are combined without degrading correlation of pixel values between the undecoded pixel and the decoding target pixel. Thereby, it is possible to accurately decode a coded image signal that has been coded by a coding method in which an adaptive pixel-by-pixel coding process and a block-by-block coding process are combined.
According to the present invention, in the image decoding apparatus, the pixel value replacing means is constructed so as to employ, as a pseudo pixel value of the undecoded pixel, a pixel value of a decoded pixel which is positioned at the shortest spatial distance from the undecoded pixel.
Since, in the image decoding apparatus thus constructed, the pixel value of the decoded pixel which is the nearest to the undecoded pixel is employed as the pseudo pixel value of the undecoded pixel, the correlation between the pseudo pixel value of the undecoded pixel and the pixel values of the peripheral pixels can be improved.
According to the present invention, in the image decoding apparatus, the pixel value replacing means is constructed so as to employ, as a pseudo pixel value of the undecoded pixel, a pixel value of a decoded pixel which is positioned at the shortest spatial distance from the undecoded pixel and on the same horizontal scanning line as the undecoded pixel.
Since, in the image decoding apparatus thus constructed, the pixel value of the decoded pixel which is the nearest to the undecoded pixel and on the same horizontal scanning line as the undecoded pixel is employed as the pseudo pixel value of the undecoded pixel, the pseudo pixel value of the undecoded pixel can be obtained by a simple method, such as to hold the pixel value of the decoded pixel for a prescribed period of time.
According to the present invention, in the image decoding apparatus, the decoding means is constructed so as to comprise a prediction value generator for generating a prediction pixel value for the decoding target pixel, on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel; and a decoder for generating a decoded pixel value by decoding the pixel value of the decoding target pixel, and adding the prediction pixel value of the decoding target value to the decoded pixel value.
Since, in the image decoding apparatus thus constructed, the prediction pixel value of the decoding target pixel is generated from the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel, and the prediction pixel value is added to the decoded pixel value of the decoding target pixel, it is possible to accurately decode a coded image signal that is obtained by coding a difference value between a pixel value of a coding target pixel and its prediction pixel value.
According to the present invention, in the image decoding apparatus, the decoding means is constructed so as to select code words for decoding the pixel value of the decoding target pixel, on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel.
Since, in the image decoding apparatus thus constructed, code words for decoding the pixel value of the decoding target pixel are selected on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel, it is possible to accurately decode a coded image signal that is obtained in a highly efficient coding process in which the code words are adaptively changed pixel by pixel.
According to the present invention, in the image decoding apparatus, the decoding means is constructed so as to select a probability table corresponding to codes for arithmetically decoding the pixel value of the decoding target pixel, on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel, and to perform an arithmetic decoding process for the decoding target pixel on the basis of the selected probability table.
Since, in the image decoding apparatus thus constructed, a probability table corresponding to arithmetic codes for decoding the decoding target pixel is selected on the basis of the pixel value of the decoded pixel and the pseudo pixel value of the undecoded pixel, and decoding of the pixel value for the decoding target pixel is performed, it is possible to accurately decode a coded image signal that is obtained in an arithmetic coding process in which the probability table is adaptively changed pixel by pixel.
An image coding method according to the present invention is an image coding method for performing, for each block comprising a prescribed number of pixels, a coding process in which pixel values constituting an image signal are successively coded on the basis of pixel values of plural peripheral pixels positioned in the vicinity of a coding target pixel, wherein with respect to a coded pixel among the plural peripheral pixels, its pixel value is set as a reference pixel value and, with respect to an uncoded pixel among the plural peripheral pixels, a pseudo pixel value, which is obtained from a pixel value of a coded pixel among the peripheral pixels on the basis of a prescribed rule, is set as a reference pixel value; and the pixel value of the coding target pixel is coded on the basis of the reference pixel values set for the plural peripheral pixels for the coding target pixel, thereby generating a coded image signal corresponding to the image signal.
Since, in the image coding method thus constructed, the image signal is blocked correspondingly to respective blocks on a single image display region, and the pixel value of the coding target pixel in each block is coded with reference to the pixel values of the peripheral pixels, and at this time, when the peripheral pixel is a coded pixel, its pixel value is referred to, and when the peripheral pixel is an uncoded pixel, a pseudo pixel value that is obtained from a pixel value of a coded pixel is referred to in place of its pixel value, in the coding process, even though the coding target pixel abuts on the block boundary and the plural reference peripheral pixels include an uncoded pixel, a pseudo pixel value having an improved correlation with the pixel values of the other peripheral pixels can be referred to as a pixel value of the uncoded pixel. Since this pseudo pixel value is obtained from a pixel value of a coded pixel in the vicinity of the coding target value, on the decoder side, with respect to an undecoded pixel to be referred to when a decoding target pixel is decoded, a pseudo pixel value obtained from a pixel value of a decoded pixel can be referred to in place of its pixel value.
Therefore, it is possible to combine an adaptive pixel-by-pixel coding process and a block-by-block coding process without degrading correlation of pixel values between the uncoded pixel and the coding target pixel, with avoiding that decoding of a coded signal becomes difficult on the decoding side.
Thereby, influence of transmission error can be converged block by block, and the coding efficiency can be improved as compared with the simple block-by-block coding process, and further, a decoding process for a coded signal without degrading the prediction efficiency of the coding target pixel can be performed accurately.
An image decoding method according to the present invention is an image decoding method for decoding, block by block, a coded image signal which is obtained by performing, for each block comprising a prescribed number of pixels, a process in which pixel values constituting an image signal are successively coded on the basis of pixel values of plural peripheral pixels positioned in the vicinity of a coding target pixel, wherein with respect to a decoded pixel among plural peripheral pixels, its pixel value is set as a reference pixel value and, with respect to an undecoded pixel among the plural peripheral pixels, a pseudo pixel value, which is obtained from a pixel value of a decoded pixel among the peripheral pixels on the basis of a prescribed rule, is set as a reference pixel value; and a pixel value of a decoding target pixel is decoded on the basis of the reference pixel values set for the plural peripheral pixels for the decoding target pixel, thereby generating a decoded image signal corresponding to the coded image signal.
Since, in the image decoding method thus constructed, a process in which the pixel value of the decoding target pixel in each block is successively decoded with reference to the pixel values of the peripheral pixels, is performed for each block comprising plural pixels, and at this time, when the peripheral pixel is a decoded pixel, its pixel value is referred to, and when the peripheral pixel is an undecoded pixel, a pseudo pixel value that is obtained from a pixel value of a decoded pixel is referred to in place of its pixel value, in the decoding process, even though the decoding target pixel abuts on the block boundary and the plural reference peripheral pixels include an undecoded pixel, a pseudo pixel value having an improved correlation with the pixel values of the other peripheral pixels can be referred to as a pixel value of the undecoded pixel.
Therefore, it is possible to realize a decoding method in which an adaptive pixel-by-pixel decoding process and a block-by-block decoding process are combined without degrading correlation of pixel values between the undecoded pixel and the decoding target pixel. Thereby, it is possible to accurately decode a coded image signal that has been coded by a coding method in which an adaptive pixel-by-pixel coding process and a block-by-block coding process are combined.
A data recording medium according to the present invention is a data recording medium containing a program for performing an image signal coding process or decoding process, wherein the program is an image processing program for making a computer perform processing of an image signal by the image coding method.
Employing this data recording medium, error propagation when a transmission error occurs can be converged block by block, and the coding efficiency can be improved as compared with the simple block-by-block coding process, and further, it is possible to accurately perform a decoding process for a coded signal without degrading a prediction efficiency of a coding target pixel, and to accurately decode a coded image signal that has been coded by a coding method in which an adaptive pixel-by-pixel coding process and a block-by-block coding process are combined.